The ornamental design for a set of stackable baseball dolls, as shown and described.
FIG. 1 is a front view of a set of stackable baseball dolls showing my new design;
FIG. 2 is a rear view of the set of stackable baseball dolls;
FIG. 3 is a side view of the set of stackable baseball dolls; and,
FIG. 4 is an opposite side view of the set of stackable baseball dolls.
The broken lines in the drawings showing the doll's face, the logo on the hat and the logo, lines, name and number on the baseball uniform are for illustrative purposes only and do not form part of the claimed design.